1. Field of the Invention
The present invention relates to a copper interconnection structure and its fabrication method and, more particularly, to a copper interconnection structure of a increasing Cu interconnection lifetime, and its fabrication method.
2. Description of the Prior Art
It is well known that in recent years efforts have been made to achieve high performance and functionality of a semiconductor integrated circuit used for an information electronic equipment such as a cell phone.
Such a semiconductor integrated circuit has many circuit elements, for example transistors. It is also a well-known fact that such a semiconductor integrated circuit is fabricated by using a high-precision semiconductor fabrication process. Further, in the high-precision semiconductor fabrication process, an interconnection structure of an increased interconnection lifetime, especially a copper interconnection structure attracts attention.
FIGS. 1A to 1D are sectional views of a copper interconnection illustrating a conventional copper interconnection fabrication process. Referring to FIGS. 1A to 1D, the conventional copper interconnection structure is provided with a barrier metal (Ta) layer 12 made of mainly high melting point metal such as Ta on an insulating layer 11, a thin seed Cu layer 14 fabricated by sputtering, a Cu interconnection layer 16a fabricated using a method such as electro-plating to thickly deposit Cu, and an SiN layer 17 fabricated by using sputtering to deposit SiN or the like.
Next, a conventional copper interconnection fabrication method will be described by referring to the sectional views of FIGS. 1A to 1D again.
First, according to the conventional copper interconnection fabrication process, as shown in FIG. 1A, a Cu interconnection groove 10 is fabricated on the insulating layer 11, then the barrier metal (Ta) layer 12 mainly made of high melting point metal such as Ta is fabricated thin by sputtering and, further as shown in FIG. 1B, seed Cu is thinly sputtered to fabricate the seed Cu layer 14. Subsequently, Cu is thickly deposited by a method such as electro-plating to fabricate the Cu layer 16. This is subjected to heat treatment at about 400° C. for 10 min. to several hours according to an interconnection thickness and/or an interconnection width to grow Cu grains, and uniformly fill the groove. Subsequently, as shown in FIG. 1C, the layer is made flat by the chemical mechanical polishing (CMP) method or the like to fabricate an interconnection 16a. A surface of the interconnection 16a is treated such as cleaning and/or plasma irradiation to remove a Cu natural oxide layer and, then, as shown in FIG. 1D, an insulating film, such as SiN or the like, is deposited by sputtering to fabricate the SiN layer 17.
As an example of an electromigration (EM) suppression in an Al interconnection, there is an example of adding a small amount of impurities such as Cu to Al as described in Japanese Patent Application Laid-Open No. Hei 08 (1996)-107110 (paragraphs 0015 to 0020, FIG. 1).
Such impurities are added because as described in “Al—Ti and Al—Ti—Si thin alloy films”, Albertus G. Dirks, Tien Tien, and Janet M. Towner, pp. 2010, “Journal of Applied Physics”, vol. 59-6 (1968), impurities are precipitated on a grain boundary to lower a hole density, whereby contribution of grain boundary diffusion is reduced. In the Cu interconnection, an EM main diffusion path is considered to be an interface between Cu and other materials. Accordingly, interface holes must be selectively removed.
However, in such a copper interconnection fabrication method, there are problems that an EM resistance of the copper interconnection is low, and particularly, in a case of the copper interconnection width is narrow, the copper interconnection has shorter lifetime than the Al interconnection. A reason is that in the case of the Al interconnection, if an interconnection width is smaller than an average Al grain size, and a gain boundary becomes a bamboo structure, Al lattice diffusion becomes a main diffusion mechanism. This lattice diffusion is much slower than either grain boundary or interface diffusion. Thus, in the case of the narrow interconnection where grain boundary diffusion is dominant which achieves a bamboo grain boundary structure, an EM lifetime is longer than a wide interconnection.
On the other hand, in the case of the Cu interconnection, even if an interconnection width is smaller than an average Cu grain size, and a grain boundary becomes a bamboo structure, not Cu lattice diffusion but interface diffusion becomes a main diffusion mechanism. Thus, an increase in the EM lifetime observed in the case of the thin Al interconnection is not seen in the case of the Cu interconnection. As a result, in the case of an interconnection width is small, the Cu interconnection has shorter lifetime than the Al interconnection.